- use BASH read's prompt capability + use $REPLY rather than polluting
  variable space + use -n var-is-not-empty test flag instead of negating the -z
  var-is-empty flag
Use read -s (noecho) and -p message to ask for passphrase
Replace destroy() with instructions on how to remove a container as it wasn't working anyway.
Make /tmp actually be temporary (tmpfs).
Merge pull request #53 from specing/misc_fixes

Misc fixes (readline, destroy, tmpfs mount, etc.)
Rewrite ugly return-value checking code
Refactor fetch_template into stage3 and ptree fetching functions
Use bashisms instead of ifs and make ARCHVARIANT lowercase and local
rationale: ARCHVARIANT is modified for local purpuses only and only
environment variables should ever be uppercase.
do the same to STAGE3URL + don't use {} in parameter expansion unless it
is not obvious what is expanded.
print a newline because the one after the password gets eaten by noecho
same for STAGE3LATESTSUBPATHURL
same for LATEST_STAGE3_SUBPATH
be more informative about what we are downloading
exit immediately on error
same for OUTPUT_FILE
make tar autodetect archive compression
same for OUTPUT_DIR
+ use Bash internals to determine the directory name instead of calling
external programs.
+ mkdir shouldn't print anything when invoked with -p.
same for INPUT_URL
Make stage3 downloading more informative
alias wget rather than keeping it in a variable (good coding practices)
Remove bz2 -j flag and add -p preserve permissions.
Additionally, tar shouldn't print anything unless something goes wrong,
which the user may be interested it.
Useless variable is useless.
Add execute_exclusively() helper function